US Director of National Intelligence John Ratcliffe has said that Russia and Iran have both tried to interfere with the 2020 presidential election.
Mr Ratcliffe made the announcements at a hastily arranged news conference that also included FBI Director Chris Wray.
The announcement two weeks before the election showed the level of alarm among top US officials that foreign actors were seeking to undermine Americans' confidence in the integrity of the vote and spread misinformation in an attempt to sway its outcome.
"We have confirmed that some voter registration information has been obtained by Iran, and separately, by Russia," Mr Ratcliffe said during the news conference.
Most of that voter registration is public.
But Mr Ratcliffe said that government officials "have already seen Iran sending spoofed emails designed to intimidate voters, incite social unrest and damage President Trump".
Mr Ratcliffe was referring to emails sent yesterday and designed to look like they came from the pro-Trump Proud Boys group, according to government sources.
US intelligence agencies previously warned that Iran might interfere to hurt Mr Trump and that Russia was trying to help him in the election.
US Election 2020
Outside experts said that if Mr Ratcliffe was correct, Iran would be trying to make Mr Trump look bad by calling attention to support and threats by the sometimes violent group.
A spokesman for Iran's mission to the United Nations denied Iran had sought to meddle in the US election.
"Iran has no interest in interfering in the US election and no preference for the outcome," spokesman Alireza Miryousefi said in a statement.
US Senate Democratic leader Chuck Schumer, who received a classified briefing yesterday afternoon on election security, said he disagreed with Mr Ratcliffe that Iran was specifically trying to hurt Mr Trump.
"It was clear to me that the intent of Iran in this case and Russia in many more cases is to basically undermine confidence in our elections. This action I do not believe was aimed ... at discrediting President Trump," Mr Schumer told MSNBC in an interview.
White House spokesman Judd Deere said Mr Trump has directed government agencies "to proactively monitor and thwart any attempts to interfere in US elections, and because of the great work of our law enforcement agencies we have stopped an attempt by America's adversaries to undermine our elections".
The emails are under investigation, and one intelligence source said it was still unclear who was behind them.
Another government source said that US officials are investigating whether people in Iran had hacked into a Proud Boys network or website to distribute threatening materials.
This source said US officials suspect the Iranian government was involved but that the evidence remains inconclusive.
Some of those emails also contained a video, debunked by experts, that purported to show how fake ballots could be submitted.
Mr Ratcliffe said that claim was false.
The second government source said US authorities have evidence that Russia and Iran had tried to hack into voter roll data in unidentified states.
But the source added that because much of that voter data is available commercially, the hacking may have been aimed at avoiding payment.